Electrochemical devices, including proton exchange membrane fuel cells, electrolyzers, chlor-alkali separation membranes, and the like, are typically constructed from a unit referred to as a membrane electrode assembly (MEA). Such MEA's comprise one or more electrode portions, which include a catalytic electrode material such as Pt or Pd, in contact with an ion conductive membrane. Ion conductive membranes (ICMs) are used in electrochemical cells as solid electrolytes. In a typical electrochemical cell, an ICM is in contact with cathode and anode electrodes, and transports ions such as protons that are formed at the anode to the cathode, allowing a current of electrons to flow in an external circuit connecting the electrodes.
In a typical hydrogen/oxygen fuel cell, the ions to be conducted by the membrane are protons. Importantly, ICMs do not conduct electrons/electricity, since this would render the fuel cell useless, and they must be essentially impermeable to fuel gasses, such as hydrogen and oxygen. Any leakage of the gasses employed in the reaction across the MEA results in waste of the reactants and inefficiency of the cell. For that reason, the ion exchange membrane must have low or no permeability to the gasses employed in the reaction.
ICMs also find use in chlor-alkali cells wherein brine mixtures are separated to form chlorine gas and sodium hydroxide. The membrane selectively transports sodium ions while rejecting chloride ions. Such membranes may also be useful in batteries and electrochemical storage cells, particularly membranes that transport lithium ions. ICMs also can be useful for applications such as diffusion dialysis, electrodialysis, and pervaporization and vapor permeation separations. While most ICMs transport cations or protons, membranes that are transportive to anions such as OH− are known and commercially available.
Commercially-available ICMs are not entirely satisfactory in meeting the performance demands of fuel cells. For example, Nafion™ membranes (DuPont Chemicals, Inc., Wilmington, Del.), which are perfluorocarbon materials having pendent sulfonate groups, are considered expensive and structurally weak when wet. Nafion membranes are not generally available at thicknesses of less than 50 μm. While Nafion membranes with lower equivalent weight can be used to obtain lower ionic resistance, lower equivalent weight membranes are structurally weaker and thus require reinforcement.
The search for new acid-functional fluoropolymers has been impeded by the difficulty inherent in copolymerizing acid-functional fluoromonomers with tetrafluoroethylene or other suitable perfluoro comonomers.
U.S. Pat. Nos. 4,894,410 and 4,956,419 (3M) disclose the manufacture of fluoropolymer membranes having various functional groups appended through thio linkages.
U.S. Pat. No. 5,395,886 (Dow Corning) discloses a method of modifying partially-fluorinated hydrocarbon polymers to provide latent reactive substituents and polymers crosslinked by means of those substituents. The latent reactive substituents are appended by nucleophilic addition subsequent or concurrent to dehydrofluorination of the polymer. The reference does not disclose a polymer membrane sufficiently substituted with acidic functions to function as an ion conducting membrane.
U.S. Pat. No. 5,656,386 (Paul Scherrer Institut) discloses fluoropolymer membranes having various functional groups appended by a radiation grafting method.